We propose to evaluate and adopt a repeatable noninvasive rebreathing method for on-line computer determination of pulmonary capillary blood flow, pulmonary tissue volume and capillary blood volume, diffusing capacity, lung volume and capillary blood volume, diffusing capacity, lung volume, and oxygen consumption in canine models and also in patients with hypoxemic respiratory failure. We also plan to evaluate the use of positive end-expiratory ventilation in both study groups utilizing the rebreathing technique along with other physiological assessments of respiratory function including measurements of pulmonary mechanics, gas exchange and hemodynamic status. It is hoped that a clinical assessment regimen to aid in the rational management of patients can be developed.